The principles of pulsed doppler systems for detecting the presence of a moving target are applicable to radar, sonar, and other types of radiant energy, such as optical energy. However, pulsed doppler systems are usually employed in connection with radar and the ensuing description is confined to that particular art, although it is to be understood that the principles of the invention are applicable to other related arts.
A class of radar has been developed to detect the presence of a moving target in the presence of non-moving (clutter) radar echoes. Such systems are frequently termed intrusion detectors because it is desired to detect the presence of a moving target to the exclusion of stationary targets. While range and angular direction can be obtained with pulsed doppler radar systems, the derivation of this information is secondary to detecting a moving target in the presence of non-moving targets.
Several different types of systems have been developed utilizing pulsed doppler techniques to detect a moving target in the presence of non-moving targets. In one system, a simple, single antenna having an omnidirectional pattern is adequate to cover a volume of space to be protected against intrusion. However, the single antenna frequently does not provide the desired results because the antenna pattern is materially modified by obstructions. For example, pulsed radar systems are often deployed on aircraft having structures, such as tail and fuselage members, which prevent the pattern of a single antenna from having optimum gain over a large field of view.
To increase the field of view and provide greater gain, plural antennae are utilized. The several antennae frequently have overlapping patterns, each assigned to subsectors of an entire volume to be protected against intrusion. While most of the prior art systems include two antennae, it is to be understood that multiple antennae can be utilized for multiple obstacles, or for very high gain antenna arrays. In the following discussion, only two antennae are considered, but it is to be understood that the same problems are extant with more than two antennae. Similarly, when the improved system of the present invention is discussed, only two antennae are usually discussed, but it is to be understood that the principles of the invention are applicable to more than two antennae.
The multiple, prior art antennae are often connected together so they are simultaneously energized to radiate at the same time in response to a single energy pulse. When the antennae are simultaneously energized, they are also usually simultaneously responsive to a return, reflected signal from a target. To enable the antennae to cover a protected volume, it is necessary for adjacent antennae to have patterns with overlapping spatial regions. However, the overlapping regions cause interference "lobes" that can cause deep nulls in a composite pattern, with a resulting loss of reflected signal to the antenna array. This effect is well-known and is illustrated, for example, in FIG. 3.12 of "Radar Systems Analysis", published by Prentis-Hall, 1964.
To obviate the problems associated with interference "lobes", each antenna of an array is sequentially energized by a radar pulse so that antennae having overlapping patterns are not simultaneously energized. By sequentially energizing different adjacent antennae, a protected volume of interest is effectively scanned. Reflected, return signals from a target are supplied by the antennae of the array to a single receiver including a high Q, doppler filter bank of contiguous filters. The doppler filters respond to reflected pulses having a carrier frequency that is shifted from the carrier frequency of the pulse energy originally activating the antennae. A high Q, typically crystal, filter attenuates the response to each reflected pulse from the non-doppler shifted return which corresponds to the non-moving "clutter" echoes.
The problem with this approach is that stationary targets have a tendency to appear as moving targets. Switching the antennae in sequence and feeding the sequential return signals from the plural antennae to a single channel causes "clutter" from the stationary targets to be modulated with harmonics of the antenna switching frequency. These clutter harmonics pass through the doppler filter and appear as noise that severely limits the sensitivity of the system to true moving targets which produce a doppler shift in the carrier frequency.